Polyester films have many varied application sectors because they have excellent optical and mechanical properties. One application sector is sheet-metal lamination, in which the polyester film is laminated to sheet metal.
Lamination takes place in order to protect the sheet metal from corrosion, or for decorative purposes (printing). In the case of cans which are produced from film-laminated sheet metal, the film laminated to the inner side serves as barrier between packaged product and metal. The film therefore firstly inhibits diffusion of corrosive constituents of the packaged product to the metal and secondly inhibits diffusion of corrosion products into the packaged product. A film laminated to the outer side serves not only for corrosion protection but also generally for decorative purposes, a particularly effective promotional method here being reverse printing, which simultaneously protects the print.
Sheet-metal-film laminates are usually produced by, for example, combining a multilayer, sealable film with sheet metal heated to a high temperature. In another possible method, the film is adhesive-bonded to the sheet metal. Here, solvent-based adhesives or hot-melt adhesives are used. The films according to the present invention are particularly suitable for lamination by means of the hot-melt adhesive.
EP-B-474 240 describes polyester films for sheet-metal lamination which comprise particles whose average diameter is smaller than or equal to 2.5 μm. The films are comprised of a copolyester whose melting point is from 210 to 245° C. The examples mention copolyesters whose isophthalic content is from 9 to 12 mol %. The laminated sheet metal is processed to give deep-draw cans for food packaging.
EP-A-586 161 describes polyester films for lamination to sheet metal which comprise a copolyester whose melting point is from 210 to 245° C. and two particles of different size with average diameter of from 0.05 to 0.6 μm and from 0.3 to 2.5 μm. The laminated sheet metal is processed to give deep-draw cans for food packaging. The films are intended to have good heat resistance and are intended not to alter the taste of the contents.
EP-A-1 086 979 describes polyester films suitable for lamination to sheet metal via melting. The films are comprised of a copolyester which contains less than or equal to 18 mol % of isophthalic acid and porous particles whose average diameter is from 0.1 to 2.5 μm. The roughness (Ra) of the films in the examples is from 12 to 30 nm.
EP-A-1 176 163 describes films which are comprised of at least 90% by weight of a thermoplastic polyester and which comprise particles smaller than 0.1 μm and/or particles larger than 1 μm.
The films of the prior art are disadvantageous because they either provide an inadequate barrier with respect to metal ions or have excessive water-permeability, or have excessive flavor absorption. They also have inadequate mechanical strength, leading by way of example to problems in printing. Because roughness is low, the films have poor windability and sheet metal laminated to these films is difficult to remove from a stack.